1. Field of the Invention
The present invention relates to a magnetic induction, light-emitting display type proximity switch used for displaying the operation position of a fluid pressure cylinder, float, and the like.
2. Discussion of the Background
Fluid pressure cylinders including hydraulic cylinders have been used frequently for various automatic machines as a drive source. For the purpose of operation display, the fluid pressure cylinder is used in combination with, for example, magnetic induction, light-emitting display type proximity switch. A conventional magnetic induction, light-emitting display type proximity switch and its application mode are shown in FIG. 8.
A proximity switch 30 is so constructed that electronic components such as a reed switch and an LED 32 are enclosed in a resin case 31, and a cord 33 is pulled out from the case to the outside of the case. When being used for a hydraulic cylinder 40, the proximity switch 30 is fixed to the outside surface of the cylinder 40 by using a band 41 or the like. The LED 32 faces the outside of the resin case 31 through an opening provided in the resin case 31 so as to be seen from the outside. On the other hand, a permanent magnet is installed on a piston of the hydraulic cylinder 40.
When the hydraulic cylinder 40 is operated, and the piston reaches the proximity switch 30, the reed switch in the resin case 31 is operated by the induction of the permanent magnet, and the LED 32 is energized, so that the piston position is displayed.
The hydraulic cylinder 40, which is used as a drive source for various automatic machines, is often placed in an atmosphere where liquids such as cutting oil and coolant are scattered. For this reason, the proximity switch 30 mounted to the hydraulic cylinder 40 is also required to have high waterproofness. From this point of view, a filler is often put in the resin case 31, which also serves as assembly fixing means for the case 31. Also, in order to further improve the waterproofness and to increase the mechanical strength, a proximity switch in which a metallic cap is put over the resin case 31 has been developed.
However, the conventional proximity switch has problems described below.
If the proximity switch is used for a long period of time in an atmosphere where liquids such as cutting oil and coolant are scattered, even in the switch in which a filler is put in the resin case 31, the entrance of liquids may cause poor insulation and therefore malfunction caused by poor insulation. This is because the LED 32 is exposed to the outside of the resin case 31 and a gap is inevitably produced here, and also a thermal change in volume and change in physical properties caused by the difference in material properties between the resin case 31 and the filler, and a change in volume and change in physical properties caused by the contact with cutting oil, coolant, and the like liquid produce a gap between the resin case 31 and the filler, or create cracks in the filler, so that the entrance of liquids from these gaps causes poor insulation. Although the proximity switch in which metallic cap is put has relatively high waterproofness, it has nevertheless a danger of poor insulation caused by the entrance of liquids. For these reasons, it has been difficult to use the conventional proximity switch in water.
The resin case 31 is high in cost, and requires manpower for assembling the case 31 and for putting the filler in the case 31, so that the cost of the resin case 31 is considerably high as compared with the costs of principal components such as the reed switch and LED 32. The proximity switch in which the metallic cap is used is especially high in cost.
The proximity switch is inconveniently made larger by the use of the resin case 31. Also, the shape of the proximity switch is limited because the resin case 31 is expensive and has a difficulty in being diversified in shape.
Since the LED 32 is exposed to only one direction from the resin case 31, and the light emission can be checked only from the exposed side in a dot form, so that it is essentially difficult to make this check, and sometimes the light emission cannot be seen when the proximity switch is used in a limited space.